Color television



May 4, 1954 R. E. MGcoY ETAL 2,677,723

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(Cl. ri-5.4)

l 15 Claims.

Our present invention relates to color television, and particularly toreceiving equipment for those systems in which the signals representingdifferent primary colors are transmitted sequentially. Its main objectis to provide electronic means for switching colors, thereby enabling acathode-ray picture tube to produce images in all colors on its screen.

Further objects are: to produce colored pictures with a relativelysimple tube structure, ern-- ploying only one picture tube and only oneelectron beam within that tube; to combine or superimpose images inthree primary colors on a single screen; to accomplish color switchingin the pieture tube itself, rather than in any auxiliary tube; toprovide means for changing colors rapidly, even as rapidly as may berequired in a dot sequential color television system; and, incidentally,to provide simple and efjcient circuits for controlling thecolor-switching means.

Generally, according to our invention, the path of the electron beamwithin the picture tube is modified slightly, at appropriate times, tochange the color of light produced when the beam strikes the screen.Ideally, the path modification should occur in abrupt steps, to makedefinite the switching from one color to the next in sequence. Thiswould require a rectangular stair-step Wave ci current or voltage forcontrol purposes; and. the ideal Waveform is unattainable, particularlyat the high frequencies required by a dot-sequential system.Accordingly, additional objects of our invention are: to provide asimple approximation to the ideal waveform; and to provide means forsuppressing the beam at moments when incorrect colors might be producedbecause of deviation (beyond tolerable limits) from the idealcolor-control Waveform.

Various forms of our invention are illustrated in the accompanyingdrawings, in which:

Figure 1 is a schematic diagram of a color-television receiver employinga picture tube with striped screen;

Figure 2 is a front View of the striped screen;

Figure 3 shows waveforms used for color-control and for blanking in adot-sequential television receiver;

Figure 4 shows schematically a stop-start oscillator, suitable forcontrol of color switching in a dot-sequential color television system;

Figure 5 is a schematic diagram of a colortelevision receiver employinga picture tube with finned screen;

Figure 6 is an exterior View of the finned screen;

Figure 7 is an enlarged view of a small section through the nned screen,between lines 36, 3l of Figure 5;

Figure 3 is a schematic diagram of an amplifier suitable for driving thecolor-switching means in a dot-sequential television receiver;

Figure 9 shows waveforms used for color switching in a line-sequentialor a field-sequential color television system.

in describing embodiments of our invention, We shall first show theirapplication to a typical dotsequential system using the three primarycolors red., blue and green, then point out the diiferences required invarious details for application to other color systems. Depending on thestandards finally adopted, the fundamental color-switching frequency fora dot-sequential system is likely to fall in the range from 3 to 9megacycles per second. For brevity, call this frequency fd. Then, duringone cycle at frequency fd, signals are transmitted to represent onepicture element or dot of each color-red, blue and green-in turn. Thesesignals are received and detected in a conventional manner.synchronizing pulses, which the transmitter periodically (during theflyback interval of each scanning cycle) substitutes for visual-datasignals, are separated and used to control the timing of the two sweepgenerators that drive horizontal and vertical deflection coils mountedoutside the neck of the cathode-ray tube (or plates inside the neck, incase of electrostatic deflection), also in a conventional manner. In theneck of the cathode-ray tube, near the base, is an electron gunproducing a beam of electrons directed past the deflection coils towardthe large end of the tube. Intensity of the beam is controlled byvideo-frequency signals from the radio receiver, applied to a controlelectrode in the electron gun.

Up to this point in the description, the features mentioned could be ofany known types suitable for their respective purposes. The particularchoice of items is no part of our invention, and details are indicatedonly as a reminder of the functions taken for granted in any televisionsystem, whether monochrome or color. To complete the receivingarrangement for monochrome television, the cathode-ray tube might have,at its large end, a uorescent screen with uniform color (preferablywhite) of fluorescence. This screen would be placed substantiallyperpendicular to the direction of the electron beam. For colortelevision according to the embodiments of our invention shown in Figure1, the cathode-ray tube will have a screen divided into sections orstripes 3 that display different colors of light when struck by theelectron beam.

Screen with horizontal stripes One form of striped screen is illustratedin Figure 2. For the sake of clarity, only a few broad stripes aredrawn; but in practice there would be several hundred narrow stripes ofeach color, a complete set of three colored stripes (red, blue andgreen) corresponding to each horizontal line in the picture-scanningraster. For example, if the color-television system follows thestandards of present monochrome systems, there will be some 500 sets ofhorizontal stripes (525, less lost during vertical retrace) in theheight allotted to the picture. To an observer viewing the screen from amoderate distance, individual stripes will be indistinguishable becauseof their ,narrcwness Stripes can be produced by using astriped colorlter on the side from which the screen is to be viewed, with a whitephosphor on the side exposed to the electron beam; or three differentphosphors can be deposited in a striped pattern, each phosphor beingchosen for its ability to produce light of a single color (red, blue orgreen). The latter course is generally preferable, since higher luminouseidiciency and greater brilliance may be expected when none of the lightneed be absorbed by filters; but if there is available a phosphor ofexceptionally high efficiency in a color slightly different from one ofthose desired, the over-all efficiency obtained by combining thatphosphor with a filter to correct its color may prove better than thatof some other phosphor-s that could produce the desired color withoutthe aid of a filter. Assuming now that the cathode-ray tube I in Figure1 contains a striped screen 2 of the type just described, the color oflight made visible depends on the position of the spot where theelectron beam strikes the screen. Under control of the conventionalsweep system,-including coils 3, 3', 4, 4', the beam scans a rastercovering the area allotted for the television picture with a series ofhorizontal lines. By appropriate adjustment of vertical-sweep amplitude,this raster can be so fitted to the pattern of horizontal stripes onscreen 2 that the normal path of the scanning spot follows the center ofeach successive stripe of a particular color-say blue. Video output ofradio receiver 5 applied to control grid Ii of cathode-ray tube I variesthe intensity of the electron beam and so the intensity of the lightproduced as the beam strikes the screen 2. The result thus far is amonochrome picture; but its color can be changed from blue to red orgreen by a slight additional vertical deflection of the electron beam.With the color sequence illustrated in Figure 2, a downward deflectionwould change the color from blue to red, while an upward deflectionwould-make the change to green. Our invention includes means forproviding such additional deflection periodically, in synchronism withcorresponding changes in the color represented by the visual-datasignals being received, so that the properv color of light will beproduced by the screen.

For the dot-sequential color system previously mentioned, the colorswitching process at the receiver must take place at the samefundamental frequency, fd, employed at the transmitter.. This frequencyis reproduced by oscillator '7, under control of synchronizing signalsthat are selected by appropriate circuits in the radio receiver 5.Oscillator "I drives a frequency doubler, 8, and an amplifier, 9.Doubler 8 also drives amplifier 9; and the resultant amplifier output(at frequencies fd and 2id) is applied to auxiliary deflection coils I0,I0', thereby causing vertical deflection of the beam as shown by graphII in Figure 3. Oscillator 1 also drives a frequency tripler, I2, andoutput of the tripler (waveform I3 in Figui-e3) is applied to controlgrid S of the cathode-ray tube, in addition to the video output of theradio receiver.

Since the deflection produced by action of the main scanning systemvaries linearly with time while the beam is sweeping across the screen,distances along the horizontal axis of Figure 3 maybe considered torepresent either time in- .tervals or the horizontal position of thespot where the beam strikes screen 2. The time interval actually shownin Figure 3 is rather short, comprising only two cycles at frequency jd,and the corresponding distance traversed by the scanning spot is smallcompared to the width of screen 2; but the same waveforms continuethroughout the scanning of each line of the raster.

The D.C. bias voltage applied between control electrode 6 and cathode I4of the picture tube is made suiicient to cut off the electron beam byitself, even without the blanking voltage provided by frequency VtriplerI2` During the negative half cycles of tripler output (downwardexcursions of curve I3 in Figure 3), the voltage at frequency Sfd helpsto suppress the electron beam; but during positive half cycles (upwardexcursions of the curve I3) it counteracts the negative bias so thatbeam current can flow in accordance with the video signal. Byappropriate phasing ofthe tripler output voltage, the periods when thebeam is on are arranged to occur vwhen the video signal receivedrepresents picture data for one or other of the three colors, whileperiods when the beam is off occur during the transitions from one colorto another. This ensures distinct separation of the signals thatrepresent dotsV of different colors; and it allows a wide tolerance onthe waveform of the auxiliary deflection that switches colors in picturetube I. During the off periods, the defiection may vary in any mannerwhatsoever, without affecting the picture. Only during on periods needthe auxiliary deection be accurately controlled.

In terms of resulting beam deflection, the second-harmonic (Zfd)component of output from amplifier 9 is made V2 to 2/3 as strong as thefundamental-frequency (fa) component; and phasing of the two componentsis so adjusted that when the fundamental changes sign, the secondharmonic does likewise. This makes the auxiliary deflectionvarycyclically in a pattern closely resembling, a flight `of stairs withthree steps. 'I'he amplitude of deflection is so adjusted that the beamposition corresponding to the flat portion of each step lies in themiddle of a different colored stripe on the screen. Graph II illustratesthe case in which .the deflection at second harmonic is half that at thefundamental frequency. Corresponding positions of three adjacent colorVstripes are shown in Figure 3 by crosshatching with a different slantfor each color. Theoretically, the vertical variation Within the centralportionfof each step Will be a minimum when the harmonic (2rd) componentof deflection is about 58% of the fundamental (fd) lbut in practice, theexactamplitude ratio is not critical.

Phase relations of the blanking voltage (curve i3) and the auxiliarydeflection (curve Il) are so adjusted that the beam is on only duringthe central portion of each deiiection step. As a result, there is verylittle vertical motion of the scanning spot while it is on any onestripe; and the spot may occupy almost the full vertical width of astripe without risk of overlapping onto adjacent stripes of differentcolor. Vertical width of the two extreme stripes in each set of threecan be made slightly greater than that of the central stripe, as shownin Figure 3, to offset the greater Vertical movement of the spot whileit is on the two extreme stripes. Then the margin of safety againstoverlap will be substantially the same for all three steps ofdeiiection.

A system employing a number of primary colors different from three canbe provided by using a screen with corresponding colored stripes,changing the frequency multiplication used to produce blanking voltage,and changing the number of harmonics combined in amplifier 9. Ingeneral, if the number of colors is n, the blanking frequency should benfs, and the auxiliary deection should have components at frequenciesincluding all multiples of fd less than the n-th. For minimum verticalvariation within the central part of each step, the relative amplitudesof the component deflections should vary inversely with theirfrequencies, the second harmonic being slightly larger than half thefundamentalfrequency component, the third harmonic slightly more thanone-third, and so on.

Screen with vertical stripes ray tube l is essentially the same asbefore, but

is rotated 90 on its axis to make the stripes in Figure 2 verticalinstead of horizontal; and stripe width is the same for all threecolors. The

required number of stripes of each color now corresponds to the numberof dots perline, rather I than to the number of lines per frame.

The nominal number of dots per line is fd/fh, multiplied by thedot-interlace ratio. The net number of dots will be somewhat smaller, of

course, because of those lost during horizontal retrace. If dotinterlace is not used (dot-interlace ratio 1:1), the horizontal sweepamplitude is so adjusted that the time in which the beam crosses a setof three different-colored stripes is l/fd. The beam is blanked out atfrequency Sfd, under control of frequency tripler l2 (or of oscillator'1, it the oscillator frequency be changed; from fa to Sfd), at thetimes when it would otherwise strike parts of two adjacent stripessimultaneously. Graphs Il and I3, in Figure 3, show the sequence ofstripe colors and blanking, though the color changes in this case areaccomplished by motion along a linear path rather than a sinuous one.

To provide dot interlace with 2:1 interlace ratio, color sequence of thestripes is reversed on the screen, and the number of stripes and thesweep amplitude are so adjusted that the beam crosses six stripes (twosets of three) in a time interval l/fd. With the blanking frequency Bfd,as before, the beam is blanked out at the centers of alternate stripes.The remaining stripes, forming a group with normal color sequence,function as just explained for operation without dot interlace; but insuccessive sweeps over the same horizontal line the polarity of blankingvoltage will be reversed (as explained below, in connection withsynchronization), changing it from the form shown by graph I3 to thatshown by graph 33, and the alternate stripes come into use.

The stripes impose limits on the dimensions o' the spot covered by theelectron beam when it strikes the screen. Without interlace, the spotcan have as much as half the width of a stripe and yet not overlap ontoeither adjoining stripe while the beam is unblanked. With dot interlace,the spot might reach the boundary between two stripes and overlap bothstripes before blanking cuts off the beam. Overlap can be prevented byintroducing black stripes between the colored ones and making the spotwidth no more than the width of a black stripe, or by making theblanking voltage so large that the beam is extinguished just before itsedge crosses the boun- ClaryY between colored stripes. n any case, thespot width must be less than the width of a colored stripe if falsecolors are to be prevented entirely. Fortunately, a small amount ofovern lapping can be tolerated, so long as it occurs symmetrically,since its effect then is merely to make the colored image look pale, asif the primary colors were partly replaced by white light; for example,extreme overlap of the scanning spot would make the image of red lookpink.

Since the width of a colored stripe is less than the height or width ofa 3-colored picture element, the scanning spot might well be madeoblong, with dimensions lengthwise of the stripes 3 to d times itsmaximum dimension parallel to the width of a stripe. `Under any givenoperating conditions, only a limited current Vdensity can be readilyattained in the electron beam. Accordingly, use of an oblong spot,aligned with the stripes on the screen and covering substantially allthe space allowable for it, leads to maximum beam current. This providesmaximum brightness of the picture, besides distributing the light overthe screen in a way that tends to minimize flicker. `Suitable elongationof the spot can be produced simply by changing the shape of apertures inthe electron gun, or generally making the gun elliptical where itordinarily would be made circular.

Fir/med screen A preferred embodiment of our invention is shownschematically in Figure 5. The cathoderay tube in this case has a nnedscreen, 34; and two sets of auxiliary deflection coils are used with it:a small set, I0, i, near the coils of the conventional sweep system, anda large set, 35, 35', closer to the screen.

Structure of the finned screen is shown more clearly in Figure 6, afront View, and in Figure 7, an enlarged view of a small section betweenlines 36, 3l in Figure 5. At the front of the screen is a transparentplate, 3l?, which carries on its rear face a large number of opaquefins, 38, 39', etc. IThese fins are attached to a stiif fra-me, 38,around the edges of the screen, as lwell as to plate 38. The combinationkeeps them evenly spaced and parallel. Between fins, the rear face ofplate 38 has a phosphorescent coating 40, chosen for its ability toproduce light of one vcolorsay blue-when .struck by the electron beam.On the left face of each n is a phosphorescent coating, 4I, chosen toproduce light of another colorsay red. On the right face Aof each iin isa phosphorescent coating, 42, chosen to produce light of the vthirdprimary colorsay green. Stretched across the rear edges of the ns is ametal nlm, 43, thin enough to be penetrated readily by the electronbeam, but thick enough to form an ecient reflector for the light fromthe phosphorescent coatings.

One method of constructing such a screen is as follows: Prepare a greatmany sheets of thin metal by cutting them to a width slightly greaterthan the desired height (or width, if the screen is to have horizontalfins) of the screen, and to some convenient length. Coat these sheets(by any of the known methods) with phosphors of two different colors,one on each side. Coating may be done either before ogr after cutting.Stack the phosphor-coated sheets in a neat pile, separating them with alike number of spacers. These temporary spacers should be made of somematerial such as wax that can be removed later without damaging thephosphor coatings-for example, material that can be dissolved or meltedwithout dissolving the phosphors, or that can be evaporated or burned(without ash) at a moderate temperature. When the stack reaches vaheight slightly greater than the desired width (or height) of thescreen, clamp it between two sturdy plates somewhat narrower thanitself, and compress it to the exact height required. Place a metalplate against each long side of the stack, and Weld or solder it to theadjacent edges of all the metal sheets. This permanently establishes thespacing of the metal sheets, at the edges mentioned. Remove the clampingplates. At this stage the side plates should extend above and below thestack a distance approximating their own thickness. Fit two similarplates tightly into the space between the projecting edges and weld orsolder them to the side plates. This pulls each sheet taut between thetwo side plates to which its opposite edges are attached. Now slice thestack, very much as if it were a loaf of bread and the outer plates wereits crust. Make the thickness of each slice equal the desired Width(front to back) of ns in the screen. In a typical case, the slices fromone stack may provide iins for more than a hundred screens; and thoughthe labor of assembling the stack is considerable, the proportionateshare charged against each screen will be tolerably small. If a curvedscreen is desired, the slice can be cut (with a band saw, or the like)to a suitable curvature in the plane of the sheets; and individualscreens can be bent later (after plate 38 is attached), to producecurvature in what was originally the plane of the side plates. Eachslice has the form of a grille, with a rectangular frame (48) enclosinga set of iins (3S, 33', etc.). Cut plates of transparent plastic to asize matching each set of fins. Either before or after cutting, coat oneside of each such plate (38) with phosphor of the desired color. Grindor etch the exposed edges of each set of ns, if necessary, to removeburrs; then place it against the coated side of a plastic plate,applying heat and pressure until the edges of the fins cut into theplastic slightly and become imbedded to a. depth greater than their ownthickness. After thus fastening the edges of ns to plate 38, whichpermanently establishes their spacing, remove the temporary spacers frombetween them. If the phosphor coatings show any flaws, upon 8 inspectionat this stage, the defective screens can be repaired by washing off theoriginal phosphor and applying a new coat, using a process of settlingfroman aqueous suspension. Suspended particles of phosphor will settleonly on the upper side o'any surface exposed to them; and by appropriateorientation of the screen during the settlement period (with somesurfaces vertical, or horizontal but face down, to avoid the settlingparticles), the principal surfaces can be coated independently. When allthree coatings appear satisfactory, turn the rear face of the screenupward and lay over the exposed edges of the fins a very thin (say 5 or'7 millionths of an inch) plastic iilm lili. This may be appliedconveniently with the aid of water: submerge the screen, in as littlewater as possible; place a few drops of plastic solution on the water,where it will spread out evenly and form a nlm as its solventevaporates; then drain or evaporate the water until the iilm settlesgently to the desired position. Dry the screen, and mount it in a vacuumchamber. Deposit on the rear face of plastic lm 44, by evaporation froma heated wire, a thin lm of aluminum (or other metal), d3. Cement thefour corners of the screen to the glass faceplate of cathode-ray tube I,on what is to be the inside, and weld the faceplate to the rest of thetube envelope. (This may be done before metalizing the screen; then thetube envelope will serve as the vacuum chamber during formation of metalnlm 43.) Subsequent processing ci the tube, particularly the outgassingbefore it is finally sealed, may well be allowed to remove most ofplastic lm 44 since that nlm is needed only to provide a smooth surfaceon which to form metal iilm 43.

The foregoing description applies to a screen suitable for viewing fromthe side not exposed to the cathode-ray. As an alternative, plate 38might be made of metal, welded to the ns; and metal iilm 43 (and with itplastic nlm 44) might be omitted. This form of screen would be suitablefor viewing-from the side exposed to the cathode-ray.

When there is no current flowing through auxiliary deflection coils I,l0 and 35, 35', the electron beam in cathode-ray tube I arrives atscreen 34 traveling in a direction substantially perpendicular to plate38 and edgewise with respect to the fins (39, 39'). The beam electronsthen pass straight between pairs of adjacent fins and strike thephosphorescent coating 4S on the rear face of plate 38, producing a spotof blue light. The position of this spot on the screen is controlled bycurents passing through deflection coilsi3, 3', 4, 4 of a conventionalsweep system, while its intensity is controlled by the video-frequencyoutput of radio receiver 5, which is applied to control grid 6 in theelectron gun of cathode-ray tube l. Thus the result would be apictureron the screen in a single color (blue).

When current does flow through the auxiliary deflection coils, thecurrents through the two sets of coils (I0, l0' and 35, 35') are of suchpolarity and so proportioned that they produce no net change in theposition at which the electron beam strikes screen 34'; but thecombination of two opposite deflections in different regions along thepath of the beam changes its direction, its angle of incidence. Supposethe beam, initially directed toward screen 34 along the path indicatedby dotted line 45 in Figure 5, is deflected to the left as a result ofcurrent in coils It, I0'. Following a path shown by dotted line 46, itmoves progressively farther to the left of path 45 until it comes underthe influence of coils 35, 35'; then, being strongly deflected to theright as a result of the current in coils 35, 35', it turns to intersectits original path i5 in screen 34. With sufficient current flowingthrough the auxiliary deflection coils, the angle between paths t and 45can be made fairly large. In particular, it can be made so large (about8 or 9 degrees, for the proportions illustrated in Figure 7) that thebeam following path i6 strikes only the left faces of the fins it meetsat the screen, and never reaches plate 38. Thus a spot of red light isproduced, instead of a spot of blue light. In a similar manner, withcurrents of opposite polarity in the auxiliary deflection coils, thebeam path can be modified to that shown by dotted line lil; then theelectrons in the beam strike only the right faces of fins, and onlygreen light is produced. The slight fore and aft shift of spot positionis partly masked by scattering and reflection of light, within the boxformed by twoadjacent fins and the nlm 153 (behind them), so that theapparent spot position moves only a small part of the distance betweenadjacent fins. Fin spacing is made no larger than the width of the spot;consequently, even to an observer some distance oil center (whereparallax might be appreciable), the spot appears in substantially thesame posi tion whether its light comes from phosphor coating 40 on plate38, from the coating 4l on lin 39, or from the coating 42 on iin 38. Byapproprin ate control of the currents through the auxiliary deflectioncoils, colors can be switched in any sequence and with any timing likelyto be required for reproduction of a colored picture.

The two sets of auxiliary deflection coils must have their magnetic axesoriented to make the directions of their individual deflectionssubstantially opposite, and approximately perpendicular to the ns ofscreen 34, but not necessarily either horizontal or vertical. If theflns are horizontal, their number may be chosen to match the number ofhorizontal lines used in scanning the picture, so that as the scanningspot passes across the screen it will follow the center of eachsuccessive space between adjacent ns; then if the spot is not much widerthan the fin spacing, only a small part of the electron beam will bewasted by striking the rear edges of fins, and that fraction will besubstantially the same no matter which line is being scanned. Similarly,if the fins are vertical, their number may be chosen to match the numberof 3-color dots in a line, so that the position of the spot relative toa pair of adjacent ilns will be the same for each successive dot of anyparticular color. Such arrangements permit use of the minimum number ofilns, but require adn justment of the scanning raster to a definiteheight or width determined by the n spacing. Alternatively, the numberof fins may be made much larger (preferably by at least a factor of two)than the number of horizontal lines or the number of 3-color dots perline; then if the scanning spot is large enough to make full use of thescreen space available for each dot, it will always straddle at leastone lin; and though the fraction of beam current wasted on the rearedges of fins will vary slightly as the scanning spot sweeps across thescreen, the corresponding variation of brightness will be too slight forthe human eye to distinguish. When the screen is designed according tothis latter alternative, the fins may cross the screen at any convenientangle, since their direction does not appreciably affect the 10appearance of the picture on the screen. We prefer to make the ilnsvertical, since they will tend to bend or sag (from their own weight) ifarranged horizontally or diagonally.

In a typical 10" screen (picture size 6" x 8f', according to present U.S. standards) there might be 800 to 1,000 fins spaced 0.01" to 0.008apart, each 6 long (vertically), 0,1 wide (front to back) and 0.001 to0.002" thick. Individual ns will be very flexible (like ribbon), even ifmade of the stiffest metal known; but when pulled taut by the frame 48andbonded together (at the front by plate 3c and the back by film 43)the fins assembled into a screen form a fairly rigid structure.

In the receiving arrangement shown by Figure 5, oscillator 'l reproducesthe fundamental colorswitching frequency (fd) of the dot-sequentialsystem under consideration. It is synchronized with the transmitter bymeans which will be explained later. This oscillator drives a frequencydoubler 3, and an amplifier l). Doubler 8 also drives amplifier 9; andthe resultant oute put, at frequencies, fd and Zd, is applied to the twosets of auxiliary deflection coils (l0, l0', and 35, Oscillator 'l alsodrives a frequency tripler, I2; and tripler output is applied to thecathode, lll, of picture tube l. Alternatively, tripler output could beapplied to control grid 6, as in Figure 1. In either case, it varies thepotential difference between grid S and cathode lil at a frequency Sid,as shown by graph i3 of Figure 3. In combination with the D.C. biasvoltage applied between grid 6 and cathode I4, it

' cuts off the electron beam except for periodsof a half cycle or lessduring upward excursions of the wave, graph I3.

Graph Il of 'igure 3 shows approximately the waveform of currentsupplied by amplifier c to the auxiliary deflection coils. Since theangular deflection produced by each set of coils is proportional totheir current, the ordinates of graph I! are proportional to deflectionalso. Three relatively flat steps appear in each cycle (at frequency .ifd) of auxiliary deflection, at times when the beam is on (upwardexcursions of graph i3). For the bottom step, the beam path is bent likeline 65 of Figure 5, and the beam produces a spot of red light; for themiddle step, the beam path resembles line 45, and blue light isproduced;

while for the top step, the beam path is bent like line tl, and greenlight is produced. rihis sequence is repeated cycle after cycle, in timewith corresponding color switching at the transmitter. Choice ofdeflection waveform for this purpose is governed by two requirements:within the middle step, the maximum deflection should be kept as smallas possible; while within the other two steps, the deflection shouldexceed the minimum required to' make the ns completely shade plate 38from the electron beam. For best results, the amplitude of deflection atsecond-harmonic frequency (2id) should be made about 55% of thefundamental-frequency (fd) amplitude; this ratio keeps the middle-stepdeflection less than 2% of that within the other two steps; but thevariations will be scarcely noticeable for any ratio from to 50%.

Synchronization Color synchronization is provided by the circuit shownin Figure Il, which is one of many possible forms for oscillator l. Thisemploy-,g two electron tubes (Shown as trio-des, though tetrodes orpentodes might be used), one (il) to sustain oscillations, the other(I8) to stop them. To input terminal I9 are applied composite videosignals taken from radio receiver at a stagewhere the synchronizingpulses have positive polarity (e. g., at the input to the i'lnal videoamplifier). During each pulse, the grid 2Q of tube IS draws current,thus charging capacitor 2l. After each pulse, the capacitor starts todischarge through grid-leak resistor 22, but so slowly that it retainssuicient charge to provide a large negative grid bias; and the nextpulse will drive grid above the potential of cathode 23 only a verysmall amount while recharging capacitor 2 I. Thus the peaks ofsuccessive synchronizing pulses at grid 2U are, in effect, clamped atapproximately the potential of cathode 23. The amplitude of the appliedsignals is so adjusted that the anode 2li of tube I8 passes current onlyduring the synchronizing pulses; all picture-data signals at grid 2Sfall in the range below cut-01T, and have no effect on the anodecircuit. Anodes Ei, 25 of tubes Il and I8 are connected in parallel,both being supplied through inductance 2S from the same D.-C. source. Apiezo-electric crystal resonator 2l is connected from anode 25 to grid28 of tube Il', while grid 28 is connected through resistor 29 to aD.-C. source Sil that provides a fixed bias voltage. If tube I 8 wereabsent, this arrangement would be a conventional type ofcrystal-controlled oscillator; and between synchronizing pulses, whenanode 24 draws no current, tube Il does cause oscillations at afrequency (fd) determined primarily yby crystal 2l. The circuitparameters, and the fixed bias applied to grid 28 from source 3U, are soadjusted that tube Il maintains the oscillations at a substantiallyconstant amplitude with the tube operating class A (no grid current andno cut-off of plate current). During each synchronizing pulse, whileanode 24 of tube I8 draws current it acts as a relatively low A.C.resistance shunting tube Il. This clamps the oscillations severely; andthey dwindle to insignicance within the time normally taken by one ofthe pulses that are transmitted for horizontal-sweep synchronization. Atthe trailing edge of each synchronizing pulse, the sudden cessation ofcurrent to anode 24 (in a time less than l/ fd) not only removes thedamping effect but provides a strong impulse to restart thecrystal-controlled oscillation. Oscillator output may be takenconveniently from a secondary coil 32 coupled to plate-feed coil 26.

Assuming that a similar oscillator is used to control color switching atthe transmitting end of the television system, with a crystal resonantat substantially the same frequency, the two oscillators will startpractically in unison after each synchronizing pulse; and at the end ofa horizontal-sweep period, the phase difference between their respectiveoscillations will still be negligible, provided that the differencebetween the two oscillator frequencies is negligible in comparison tothe horizontal sweep frequency (fh). Since 1/6 cycle at frequency fd issignificant (the beam on period), the frequency tolerance isconsiderably less than fil/6. For example, with f11=15.75 kc./s.(present U. S. standard for monochrome television), a difference of 500C. P. S. would be practically imperceptible in its effect on the picturedisplayed by the receiver. Such a frequency tolerance can be maintainedeasily by crystals now available.

Any variation in the duration of successive synchronizing pulsesproduces a corresponding variation in the timing of color switching(which is controlled by the trailing edge of the synchronizing pulse, asexplained above) witlilrespectto' the horizontal sweep (which iscontrolled by the leading edge of each horizontal synchronizing pulse),and so shifts the position of color dots along each line of the scanningraster. Dot interlace can be produced simply by introduction at thetransmitter of a width-modulation of the horizontal synchronizingpulses; for example, so that alternate pulses differ in width by a time1/(2Jd) then in alternate line periods the timing of color switching andbeam blanking becomes as shown by graphs 3I and 33, instead of graphs IIand I3 respectively. With an odd number of lines per frame (e. g., 525,the present U. S. standard), the same diilerence occurs be-tweensuccessive sweeps along any given horizontal line-(at intervals of halfa frame period), so that the color dots displayed on any line of screen2 during one sweep fall midway between those displayed there during theprevious sweep. This arrangement serves to fill in the tiny gaps left by-blanking the beam between dots of different color.

The nominal number of dots of each color per line should be an integer.Without dot interlace, the number (fd/fn) might be any integer; with 2:1dot interlace the number (2m/fh) should be odd. Such a rational relationis advantageous, both at the transmitter and at the receiver, because itmakes possible several different substitutes for the synchronizedoscillator shown in Figure 4. In particular, it facilitates the use of acontinuously-operating oscillator or frequency multiplier to generatethe color-distribution frequency, fd, because it makes the phaserelation between the desired oscillations in successive line intervalsexactly what would occur naturally in case of a continuous oscillationat frequency fd. Without attempting to describe the variouspossibilities, we point out that they exist, and that the application ofour invention is not limited to receivers employing a circuit of thetype shown in Figure 4.

Auxiliary-deflection amplifier In a tube with horizontal-striped screen,power requirements of the auxiliary deflection system are very small,since the beam need only be displaced a little more than its own widthat the screen. With a finned screen, considerably more power isrequired. The angular deflections required for color selection arelarger; there are two sets of auxiliary coils instead of one; themagnetic eld of the second set (35, 35') must be stronger than that ofthe rst (I0, IU), producing a larger angular deflection of the beam; andbeing near the large end of tube I, coils 35, 35 have a relatively largespace to energize, which further increases their power needs. In atypical case, coils 35, 35' may require 100 times as many ampere-turnsas coils I0, I0', to maintain the proper ratio of deflection; and withfrequency fd in the neighborhood of 4 mc./s., the power dissipated dueto resistance of coils 35, 35 may be 20 or 30 watts even though thecorresponding power for coils II), I0 is only 3 or 4 milliwatts. Thetotal power requirements then are large enough to warrant carefulconsideration of circuit efliciency. High-frequency coils must haverelatively few turns, and therefore require relatively large currents toproduce the desired deflections. The tubes of amplifier 9, on the otherhand, are low-current devices; but the coil requirements may be matchedto the amplifier capabilities by an arrangement such as that shown inFigure 8, where auxiliary deec- 13 tion coils IIJ, lll', 35, 35' areconnected as one inductive branch of a double-tuned circuit.

Figure 8 shows all of these coils connected in series, but that is notan essential feature. Among other possibilities, they all might beconnected in parallel; coils il! and I might be connected in parallelwith each other but in series with coils 3e and 35'; or coils l0 and IBmight be connected to taps on coils 35 and 35. Because of differences incurrent distribution, each combination or" connections will require adifferent coil design to produce the proper relation between the twoopposite deflections; but the combination of coils may be treated as onecoil so far as the amplier is concerned.

The combination of coils il), l0', 35, 35' is connected from thepositive terminal of D.C. power source e9 to the anode of tube 50.Another coil, 5l, having substantially the same inductance (and,incidentally, resistance) as the combination of auxiliary-deflectioncoils, is connected from the same D.-C. power source to the anode ofamplifier tube 52. Capacitances 53 and 54, which are substantiallyequal, tune the two equal inductances to the same frequencyin this case,Zfd. Capacitance 55 tunes the push-pull combination of the twopreviously-mentioned tuned circuits to a lower frequency, and isadjusted to make that lower frequency fd. Thus for `push-pull operationof amplifier tubes 5U and 52 their combined anode-load circuit presentsa high resistance at frequency fd, while for parallel operation of thetwo tubes (capacitor 55 being inactive then because both its terminalsare at the same potential) each side of the anode-load circuit presentsa high resistance at frequency 212i. Oscillator 1 (Figure 1 or Figure 5)energizes at frequency fd a primary coil 6l) coupled magnetically tocenter-tapped secondary coil 5S. The latter is tuned to frequency fd bycapacitor and its voltage is applied in a push-pull arrangement to thecontrol grids of tubes 50 and 52. Frequency doubler 8 energizes atfrequency 2fd a primary coil 63 coupled magnetically to a secondary coil52. The latter is tuned to frequency 2fd by capacitor 6l, and itsvoltage is applied through the center-tap of coil 59 to the two controlgrids in parallel.

By the arrangement just described, the ampliner tubes are made to takefull advantage of the double resonance (at frequencies fd and 2fd) oftheir anode-load circuit, and produce a large voltage at each frequencyacross the auxiliary deflection coils iii, I8', 35 and 35. Because ofresonance, the coil currents greatly exceed the anode currents of tubes5i! and 52; and this gain (some 10Q-fold) more than compensates for thefact that approximately hah:` the power output of the amp-liner, atfrequencies fd and 2id, is dissipated in coil 5i. In general, tuning theanode circuit of ampliner e is advantageous when auxiliary deflectionrequires more current than can be supplied by a reasonably small ampliertube; otherwise a single-tube amplifier would suffice, and tube 52, coiland capacitors 53, 54, -55 could be eliminated. When the currentrequirements are high enough to make tuning advantageous, but the powerrequirements are relatively low, either tube 559 or tube 52 may beomitted from the circuit of Figure 8. If the remaining tube will enduresuch treatment, re-adiustment of its grid voltages (D.C. and A.-C.) todouble its A.C. plate current will enable it to produce substantiallythe same output as the two tubes would.

Other applications Besides the dot-sequential system outlined above,there have been proposed television systems in which the -colors areswitched vat much lower frequencies. In e, line-sequential system, thetransmitted signals represent picture data for a horizontal line of eachprimary color in turn, so that the time interval devoted continuously toone color is the sameas the interval required vfor scanning once acrossthe width of the picture. In a field-sequential system, the transmittedsignals represent picture data for a field of each primary color inturn, the interval devoted continuously to one ycolor being the same asthe interval required for scanning once from top to bottom of thepicture. Fundamental frequencies of color switching for typical examplesof these systems are 15,750 and 20 cycles per second, respectively; thatis, 15,750 sets of 3 lines per second, or 20 sets of three iields persecond, with one line or iiel'd of each primary color in each set ofthree.

With e, line-sequential system, the arrangement shown in Figure 1 can besimpliiied considerably: all the circuits involving frequencies fd, 2idor Bid can be eliminated; and auxiliary deflection coils Hl, I ll can beeliminated too. For a specified number of S-colored lines in thepicture, the horizontal-sweep frequency in this case will be three timesthe value required in either dot-sequential or monochrome systems; andthe scanning raster will have three times the usual number of lines. Byappropriate adjustment of vertical-sweep amplitude, this raster can beso fitted to the stripes of the screen that the path of the scanningspot follows the centerline of each successive stripe. Since successivestripes have different colors, in exactly the sequence required forcolor switching, the picture produced on the screen will follow thecolor scheme of the transmitted picture-data signals (which are detectedand applied to control grid of cathoderay tube I, in a conventionalmanner). Any of the well-known methods of position adjustment (e. g.,D.-C. bias applied to vertical deflection coils I, 4) can be used, ifnecessary, to correct the phase of color switching, so that (forexample) the beam strikes a blue stripe rather than a red or a greenstripe when the transmitted signal represents blue. For systems based onmore than three colors, the same principles may be employed withcorrespondingly more colored stripes. Line interlace may be used, as inmonochrome systems, provided the interlace ratio and the numbei' ofcolors are relatively prime (no common factors).

The same simplification of Figure l is possible with a held-sequentialsystem, if the standards adopted for that system provide a color-lineinterlace ratio that is a multiple of the number o-f colors; forexample, 3:1 or 6:1 in a three-color system. For a 3:1 ratio, thenominal number of lines per eld (fh/jv) differs by 1/3 from the nearestinteger'. Then the lines scanned in one field fall `between. linesscanned in the two preceding flelds, with a relative displacement 1A@the spacing of lines in each field. By proper adjustment of thevertical-Sweep amplitude and vertical centering, the raster can be sofitted to the stripes on the screen that the path of the beam sweepsalong the center line of each successive stripe of a certain color inone eld, each successive stripe oi another color in the second field,and each stripe of the third color in the final eld of each frame. For a6:1 interlace ratio, the

nominal numberof lines per neld (fil/jv) differs 4by 1/6 from thenearest integer, and there sii; fields per frame. The path of thescanning spot follows the center lines of the odd stripes of a differentcolor in each of three successive fiel-ds, 5 then follows the evenstripes of corresponding colors in the next three fields.

In a field-sequential system not using sweepfrequency ratios asconvenient as those ust nientionedfthe regular scanning arrangement willnot accomplish color switching automatically, even in a picture tubewith striped screen; and auxiliary deflection becomes advantageous forthat purpose. Iowever, some simplification of Figure 1 is stillpossible. Since standard video 15 signals include blanking pulsesbetween ields, at the times that would be natural for colo-r switchn ingin a field-sequential system, no additional blanking is necessary; andtripler l2 of Figure l need have no counterpart in this case. Auxillarydeflection can be given very nearly the ideal waveform shown in Figure9, by means cf a combination of multivibrators synchronized with thevertical-sweep generator. One such combination has been shown byHuffnagel in U. S. Patent No. 2,389,979. Whether the circuit used todrive auxiliary deflection coils l0, l' be Huffnagels or some other, itcan take the place of three items in Figure 1: oscillator 1, doubler tand ampliner t. The basic time interval (T) in Figure 9 will be 3Q(i3/jv) three vertical-sweep periods, since one step of the rectangularwave occurs for each field.

Auxiliary deflection is always required for color switching in a picturetube with nnned scree' but for either a lineor a field-sequential sys- 3tem the circuit illustrated in Figure 5 can be simplified. @neitem-frequency tripler ill-can be omitted completely, since no auxiliaryblanking is required. Standard video signals already include blanlringpulses between lines and be- 40 tween fields, at the times whentransitions from one color to another would occur. Three otheritemsoscillator l', doubler 8 and amplifier 9- can be replaced by astep-wave generator operating lat the appropriate color-switchingfrequency. Figure 9 shows the ideal waveform for auxiliary deflection inthis case. et the low frequencies required in a lineor field-sequentialsystem, this wave can be approximated very closely (in practice, muchmore closely than is necessary for color-switching purposes) by acombination of unsymmetrical multivibrators. Suitable combinations havebeen shown elsewhere, and are no part of our invention. For aline-sequential system, the step-wave generator driving the auxiliarydeflection coils (it, lil', 35', 35) is synchronized with thehorizontalusweep generator; and the basic time interval (T) in Figure 9is (S/fh) three line periods. For a eldsequential system, the step-wavegenerator is synchronized with the vertical-sweep generator; andinterval T is B/fv, three field periods.

Magnetic focusing The previous discussion of paths followed by theelectron beam has assumed that the beam was focused by an electrostaticlens system. f

-magnetic focusing is employed, the beam paths become a little morecomplicated, though the end results are similar. The most commonarrangement uses a short coil, or a permanent magnet between two annularpole pieces, around the neck of the cathode-ray tube. Its magnetic eldis largely concentrated in the region between cathode and main deeotioncoils, although it does have some influence throughout the tube,Vincluding the region between deflection coils and screen. There themagnetic influence gradually twists the beam path around the axis of thetube, making it a helical arc instead of a straight line. Since theangle of twist is'substantially the same for all beam paths, it can becompensated by rotating the deflection'coils around the tube axis in theopposite direction.

When a bent path is desired, as in Figure 5, with a change of directionbut no net change in position at the screen, magnetic focusing can bearranged to serve an extra purpose, reducing by a factor of l0 or 20 thepower required for auxiliary deflection. To do this, the first set ofauxiliary deflection coils (l0, I') is mounted close to the cathode; thecenter of the focusing magnet is placed between those coils (I0, lil')and the main deflection coils (3', 3', 4-, 4') and the second set ofauxiliary deflection coils (35, 35') is placed some distance back fromthe screen 34, perhaps half way to the neck of the tube. The focusingmagnet is made fairly long, so as to produce a moderate magnetic neldthrough most of the region between the two sets of auxiliary deflectioncoils, rather than a very strong field in a shorter space. When adjustedto focus the cathode ray into a small spot on the screen, the magneticield produces more than twist in the beam path between the two sets ofauxiliary deflection coils. If the beam is deected by coils l0, I d', itfollows a helical pathmakinga little more than a half turn before itpasses coils 35, 35', andbegins to turn back toward its original(undeflected) path even before being inuenced by the latter coils;consequently, they need supply-only a relatively small deecting force inorder to neutraliae the effect of the first set on the position wherethe beam will strike the screen. The direction of the required forcegenerally will not be aligned with either the ns of screen 34 or thedirection of the corresponding force applied by coils it, it', nor yetwith the direction of transverse beam velocity at the screen; but itwill remain fixed as long as the focusingmagnet does so: and themagnitude of force required from coils 35, 35' can be made even smallerthan that from coils I0, l0.

The saving from use of magnetic focusing as just described is limited bythree factors. First, in order to reduce the deflecting force requiredfrom coils 35, 35' and stillmaintain the desired transverse velocity atthe screen, the deflecting force required of coils l0, l0' must beincreased; and there is no advantage in carrying such modication beyondthe point Where the two sets of coils require equal power from amplifierS. Second, if the deflecting force of coils l0, I0' were made toolarge,the beam might strike the wall of the tube. Third, although the deectingforce required of coils 35, 35' could be reduced by extending thehigh-intensity portion of the focusing field beyond them, this fieldmust be kept mostly within the neck of the tube to avoid materiallyreducing the deflection sensitivity for coils 3, 3', 4, 4'. Usually thefirst factor will govern design, the other two being easilyaccommodated.

While we have shown particular arrangements of our invention, many minorchanges therein can be made without departing from the spirit and scopeof the invention.

We claim:

1. In a cathode-ray picture tube, a three-color screencomprising thefollowing: A translucent base plate; a multiplicity of opaque ns,substantially parallel to each otherfwith one' edge l ofeach nn adjacentto the plate; av thin metallic film covering the gaps between fins atthe edges away from the base plate, said nlm being nearly transparent tocathode raysbut an eiiicient reflector for light; a coating onthebas'eplataf between iins, adapted to radiate light of one color whenexcited by electrons, as it willbe if cathode rays arrive in a directionnearlyedgewiseto the fins; a coating on one side of each fin adapted toradin ate light of a second color when struck-by electrons, as it willbe if cathode rays arrive from a direction nearer that side;"a coating*onvthe opposite side of each 1in adapted to radiate-light of a thirdcolor when struck by electrons, as it will be if cathode rays arrivefrom` a kdirection nearer' said opposite side; the aforesaid iins beingwide in comparison to their spacing 'so as to shade the base plate from'any Yrays whose incidence angle, with respect to the edgewise directionof the fins, exceeds some moderately small limit.

2. In a color television YsystemV employing a cathode-ray picture tube:A nned color screen, as in claim 1, placed with its fins edgewise to thenormal direction of arrival of the electron beam at the screen;deilecting means, capable of making the electronv beam deviatefrom itsnorn mal path; restoring means, active in a region between said deectingmeans vand the screen, capable of turning the deflected beam back towardits normal path; means for energizing said deilecting and restoringmeans in the proportions required to make the beam intersect its normalpath at the screen; and means for control of said energizing means tochange theangle of incidence of the beam with respect to the iins,thereby to change the color` of light emitted from the screen.

3. In a dot-sequential color television system: A cathode-ray tube withstriped three-color screen; means for deflecting the beam to scan araster with linesfalling -upon stripes of one color only; means fordeiiecting the cathode ray additionally by small amounts; yto make itimpinge upon stripes of other colors; means for generating oscillations'at the fundamental color-sequence frequency; means for doubling vthatfrequency; means" for energizing the. aforesaid additional deilectingmeans simultaneously at the fundamental frequencyia'nd at twice thatfrequency, the relative amplitude and phase of the two components beingadjusted' to' make the resultant deflection vary in a' step-wise'vmanner with three steps per fundamental cycle,`the steps being ofsuitable magnitude to bring the cathode ray successively to stripes ofthree different colors; means for tripling the fundamental frequency;and means for extinguishing the 'cathode ray at the triple frequency,`phased to make the ray-less periods occupy .the'major portion of eachsuccessive transition interval between steps of color-selectingdeflection.

4. In a dot-sequential colortelevision system: A cathode-ray tube withits"screen subdivided into long, narrow elementary areas, or stripes,each adapted to `emit light of aparticular color when excited by cathoderays, stripes of one color being co-planar while stripes ofother colorsare on iins mounted edgewise to the first; means for deflecting thecathode ray to scan a raster on the screen; means for bending the pathof the cathode ray and thereby changing its angle of incidence upon thescreen; means for generating oscillations at the fundamentalcolor-sequence frequency; frequency-doubling means excitedV by saidoscillations; means for energizing the aforesaid path-bending meanssimultaneously at the fundamental frequency and at twice thatfrequency,the relative amplitude and `phase of the two components being adjustedto make the resultant variation of incidence angle of the cathode ray atthe screen approximate a stair-step function of time, with three stepsper fundamental cycle; frequency-tripling means excited by thefundamental-frequency oscillations; Vand means for extinguishing thecathode ray at the triple frequency, phased to make the ray-less periodsoccupy a major part of each successive transition between the steps thatcorrespond to different colors at the screen. i

5. In a dot-sequential color television system: A cathode-ray tube withits screen subdivided into many elementary areas, each such area beingsubstantially as large as the screen in one direction but very narrow ina perpendicular direction, and adjoining areas being adapted to emitlight of different colors when. excited by cathode rays; means fordeflecting the cathode ray to scan a raster on the screen; means fordeiiecting the cathode ray additionally and making it strike elementaryareas of one color or another according to the magnitude and directionof such additional deflection; means for generating oscillations at thefundamentalcolor-selection frequency; frequency-doubling means excitedby said oscillations; means for energizing the aforesaid additionaldeflecting'means simultaneously at the fundamental'frequency andat twicethat frequency, the relative amplitude and phase of the two vcomponentsbeing i'adjustedf to make the resultant deflection vary in a.step-wisemanner with three steps per fundamental cycle, the steps being ofmagnitude suitable forcolor selection; frequency-tripling meansexcited'by the fundamental oscillations; and means 'for' extinguishingthe cathode rayatthe vtriplerfrequency, phased to make the ray-lessperiods occupy the majorportion of each'successive transition intervalbetween colors.

6. In a dot-sequential colortelevi'sion system where color switching iscontrolled byoscillations of a predetermined frequency'the followingarrangement for synchronizing such`oscillations at the receiver withtheir counterpart Vat the transmitter: An electron-tube including aplate, a grid and a cathode; a feed-back. circuit between plate and gridthereof' including a piezoelectric crystal, so that the tube will gen'-erate oscillations at a frequency controlled by said crystal; a secondelectron tube. having its anode and cathode connected. in 'parallelVwith those of the first tube but its control grid separate; and acapacitance-resistance coupling to that separate control grid from apoint in the receiver video circuit that 'makes available signalsincluding positive synchronizing pulses, the coupling resistance andcapacitance being of values appropriate for providing grid-leak bias tothe second tube, the amplitude of the video signals being sufficient tomake such bias prevent anode current in the second tube except duringthe positive synchronizing pulses, the effective resistance loadingimposed on the oscillator circuit by the second tube'when its anodevconducts being suiiicient to stop oscillations,Y and the abruptcessation of current to the anode of the second tube at the end of eachsynchronizing pulse being 'sufficient stimulus to initiate oscillationswith an amplitude thatY canbe maintained thereafter by the rstjtube.

7. The following arrangement for controlling independently the angle ofincidence and the position of an electron beam at its terminus: meansfor producing a transverse field, electric or magnetic, near the sourceof the beam, to deflect the beam from its initial direction; means forproducing a longitudinal magnetic field, substantially parallel to theaverage direction of the beam in the region through which the beampasses after the aforesaid deflection, so that the transverse velocityof the beam` electrons will be rotated progressively as they advance;means for adjusting the intensity of said longitudinal field to twistthe path of the deflected beam enough to intersect, at its end, the paththe beam would have followed if not deflected; means for producing asecond transverse field, farther alongv the path of the beam than thefirst, to deflect the beam after an appreciable part of the aforesaidpath-twisting action has been accomplished, so that the magnitude oftransverse displacement of the beam, at its terminus, due to the seconddeflection is not greatly reduced by the twist efect, and the seconddeflection can control the final transverse position of the beam.

8. The following arrangement for controlling independently the positionand the angle of incidence of an electron beam upon the target cr screenthat ends its flight: First means for deflecting the beam, to controlits final incidence angle; second deflecting means, to control its finalposition; means for deflecting the beam a third time, in proportion tothe first defiection but in a different direction; means for producing alongitudinal magnetic field, substantially parallel to the averagedirection of the beam between the first two deflections; means foradjusting the intensity of this magneticfield to the value requisite forfocusing the beam; the relative direction of the first and thirddeflections being adjusted to make opposite the direction, aftertwisting by the longitudinal magnetic field, of their individual effectson final beam position; and the relative magnitude of the first andthird deflections being adjusted to make their effects equal inmagnitude, so that in combination with the longitudinal magnetic fieldthey have substantially no net effect on final beam position, but onlyon the final incidence angle of the beam.

9. The following arrangement for controlling independently the positionand angle of incidence of an electron beam at the end of its flight:Means for producing a transverse selected from the group consisting ofmagnetic fields and electric fields, to defiect the beam from itsinitial direction, thereby to control its final incidence angle; meansfor producing a second transverse field selected from said group, todefiect the beam for control of its final position; means for producinga longitudinal magnetic field, substantially parallel to the averagedirection of the beam and concentrated chiefly in the region between thetwo transverse fields, whereby the path of the beam will be twistedprogressively around the longitudinal axis of the magnetic field as itadvances, but relatively little of the twist will occur after the seconddeflection; and means for adjusting the intensity of the longitudinalmagnetic field to a value that will make the twisted path of the beam,when deected by the first transverse field, intersect at its end thepath the beam would have followed in the absence of said firsttransverse field.

10. In the deflection system of a cathode-ray tube, for color televisionor the like, where deflection is required simultaneously at twofrequencies: Inductance means and capacitance means, connected inparallel, adjacent to the path of the cathode ray with the field of oneof them crossing that path; a second inductance means and a secondcapacitance means, connected in parallel with each other, and connectedat one terminal to the first-named means, but otherwise separated fromthe first named means so that their fields do not cross the path of thecathode ray; one means in each of the aforesaid pairs being adjustablefor tuning purposes, so that they can be tuned to one of the requiredfrequencies; and an adjustable reactance, connecting the two tunedcircuits aforesaid, whereby the combination may be tuned to the other ofthe required frequencies.

11. In a dot-sequential color television system: A cathode-ray tube; ascreen therein subdivided into strip-like elementary areas, adjoiningareas being adapted to emit light of different colors when excited bycathode rays, and being disposed to receive selectively cathode raysincident upon the screen at different angles; means for producing acathode ray and directing it toward said screen; means for deflectingthe ray, near its source; means for producing a magnetic fieldsubstantially parallel to the average direction of the ray, whereby thepath of the deflected ray may be twisted to meet the screen at aposition substantially independent of the aforesaid defiection, withincidence angle dependent on said deflection; means for deflecting thecathode ray again, after an appreciable part of the twisting action hasbeen accomplished; means fcr controlling the second deflection means tomake the cathode ray scan a raster on the screen; means for generatingoscillations at the fundamental color-selection frequency;frequency-doubling means excited by said oscillations; means forenergizing the first-mentioned deflecting means simultaneously at thefundamental frequency and the double frequency, thereby to vary theincidence angle of the ray upon the screen in a step-Wise mannersuitable for color selection, with three steps per fundamental cycle;frequency-tripling means excited by the fundamental oscillations; andmeans for extinguishing the cathode ray at the triple frequency, so thata ray-less period covers the major part of each successive transitionbetween colors.

12. In a color-television receiver: A cathoderay tube; adirection-sensitive color screen therein, adapted to produce selectivelylight of different colors according to the direction of incidence ofelectrons striking it; means for producing a cathode ray and directingit toward said screen; deflecting means, near the cathoderay source,controllable to deflect the ray selectively in different directionsaccording to the color to be displayed; means for producing a magneticfield directed generally along the axis of the ray, thereby twisting thepath of the de-- flected ray so that the ray'may strike the screen atsubstantially the same point as if not defiected, said magnetic fieldbeing concentrated in a region adjacent to said deflecting means; andsecond deflecting means, affecting the ray in a region between the firstdeflecting means and the screen, so as to control the transverseposition of the point at which the ray strikes the screen.

13. In a color-television receiver: A cathoderay tube; adirection-sensitive color screen therein, adapted to produce selectivelylight of dfferent colors according to the direction of incidence ofelectrons striking it; means for producing a cathode ray and directingit toward said screen; deflecting means, near the cathoderay source,controllable to deilect the ray selectively in different directionsaccording to the color to be displayed; means for producing a magneticeld directed along the undeflected path of the ray, in a regionimmediately following said deflecting means; second deflecting means, atthe screenward end of said region of concentrated magnetic eld, wherebythe ray may be directed selectively to any point on the screen; thirddeflecting means, between the second defiecting means and the screen;and means for exciting said third deecting means in a proportion anddirection fixed relative to the excitation of the fdrst deflectingmeans, so that said first and third deflections in combinationeffectively determine the direction of incidence of the ray upon thescreen but neutralize one another in regard to the position at which theray meets the screen.

14. In a television receiver: A cathode-ray tube; a direction-sensitivecolor screen therein, adapted to produce selectively light of differentcolors according to the direction of incidence of electrons striking it;means for producing an electron beam and directing it toward saidscreen; beam-deiiecting means adjacent to said beamproducing means;means for producing a magnetic field directed generally along the axisof the beam, said field being concentrated in a region traversed by thebeam immediately after said deflection; and additional beam-deflectingmeans between said region of concentrated axial magnetic field and saidscreen.

15. In a television receiver: A cathode-ray tube; a direction-sensitivecolor screen therein, adapted to produce selectively light of diierentcolors according to the direction of incidence of electrons striking it;means for producing an electron beam and directing it toward saidscreen; beam-deflecting means adjacent to said beamproducing means;means for producing a magnetic field directed generally along the axisof the beam, said field being concentrated in a region traversed by thebeam immediately after said deflection; second deflecting means,independent of the rst, between said region of axial magnetic eld andsaid screen; and third deflecting means, between said second deflectingmeans and said screen, excited in proportion to the excitation of thefirst deiiecting means, but oppositely directed.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,296,908 Crosby Sept. 29, 1942 2,416,056 Kallman Feb. 18,1947 2,431,115 Goldsmith Nov. 18, 1947 2,446,440 Swedlung Aug. 3, 19482,461,515 Bronwell Feb. 15, 1949 2,529,485 Chew Nov. 14, 1950 2,532,511Okolicsanyi Dec. 5, 1950

